GTS of course
The "best" 355 is the one that's had the major design flaws properly addressed by a mechanic with a great reputation. Look for one with SS valves, aftermarket headers, gold connector kit and new cats.
You opened a can of worms! All of the 355's are great cars. I would say, Get a 355 that is well maintained. These cars are getting old and they are expensive to repair. Find one that has been taken care of. That doesn't mean find one with low mileage. They can be the worst. The best of the lot are the low production cars. GTS only 1500 made. Challenge cars 109.
I bought the GTS since it has the looks of the berlinetta but you can remove the roof in a sunny day.
2577 GTS made, the rare one is the F1 GTS (529 made). But, as Enzo said: "Convertibles are for playboys, coupes are for serious drivers."
Enzo could not remember all the bs he gave the public and more so to his employees LOL Intellectual intercourse was his best trait...
I was leaning toward GTS for the open air aspect. But do they flex so bad over bumps that it ruins the experience? What about with top on?
Sorry to be flip, but this has been discussed 100,000 times on this site, and seemingly at least once a week. Just do a search, read up, and drive a few. There is no wrong choice with the 355, or the 348 for that matter. Both are fantastic cars. Good luck!
Correct me if I'm wrong (don't worry, I'm not) but the switch to 5.2 was because of the requirement that all cars manufactured after Jan 1, 1996 and in use in the USA have OBD II, which included the notorious post cat O2 sensors.
What's more important is to buy a car with good history and well maintained (as much as possible). 2.7, 5.2 is quite irrelevant, ugly steering wheel or F1 down to taste. Steering wheel can always be changed.